The phosphoinositide 3-kinases (PI3 kinases or PI3Ks), a family of lipid kinases, have been found to have key regulatory roles in many cellular processes including cell survival, proliferation and differentiation. As major effectors downstream of receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs), PI3Ks transduce signals from various growth factors and cytokines into intracellular massages by generating phospholipids, which activate the serine-threonine protein kinase AKT (also known as protein kinase B (PKB)) and other downstream effector pathways. The tumor suppressor or PTEN (phosphatase and tensin homologue) is the most important negative regulator of the PI3K signaling pathway (“Small-molecule inhibitors of the PI3K signaling network” Future Med. Chem., 2011, 3, 5, 549-565).
The phosphoinositide 3-kinase (PI3K) pathway is an important signal transduction pathway commonly activated in cancer. Activated PI3K pathway leads to phosphorylation of phosphatidylinositol-4,5-bisphosphate (PIP2) to generate phosphatidylinositol-3,4,5-trisphosphate (PIP3). PIP3 can be dephosphorylated by the phosphatase and tensin homolog (PTEN), which terminates PI3K signaling. The accumulation of PIP3 activates a signaling cascade starting with the phosphorylation (activation) of the protein serine-threonine kinase AKT at threonine 308 by phosphoinositide-dependent kinase 1 (PDK1). Phosphotylated AKT activates the mammalian target of rapamycin (mTOR), which leads to phosphorylation of its downstream targets.
There are three PI3K classes, with different structures and characteristics. Class I can be further subdivided into class Ia and class Ib. Class II PI3Ks are large (170-210 kDa) proteins that have a catalytic domains that mediate calcium/lipid binding in classical protein kinase C isoforms. Class III PI3Ks are typified by the yeast protein encoded by the VPS34 gene and phosphorylate only PtdIns to produce PtdIns(3)P and they are thought to regulate vesicle transport (“Targeting PI3K signaling in cancer: opportunities, challenges and limitations” Nature Review Cancer, 2009, 9, 550).
Class Ia PI3Ks (PI3Kα, PI3Kβ, PI3Kγ and PI3Kδ) comprise heterodimers between a p110 catalytic subunit (p110α, p110β, p110γ and p110δ respectively), and a p85 regulatory adapter subunit (i.e., p85α, p85β, p55δ, p55α and p50α). The catalytic p110 subunit uses ATP to phosphorylate PtdIns, PtdIns4P and PtdIns(4,5)P2. The importance of Class Ia PI3Ks in cancer was confirmed by the discovery that the PI3K catalytic subunit α-isoform gene (PIK3CA), which encodes p110α, is frequently mutated or amplified in a number of human tumors such as ovarian cancer (Campbell et al., Cancer Res., 2004, 64, 7678-7681; Levine et al., Clin. Cancer Res., 2005, 11, 2875-2878; Wang et al., Hum. Mutat., 2005, 25, 322; Lee et al., Gynecol. Oncol., 2005, 97, 26-34), cervical cancer, breast cancer (Bachman et al., Cancer Biol., Ther., 2004, 3, 772-775; Levine et al., supra; Li et al., Breast Cancer Res. Treat., 2006, 96, 91-95; Saal et al., Cancer Res., 2005, 65, 2554-2559; Samuels and Velculescu, Cell Cycle, 2004, 3, 1221-1224), colorectal cancer (Samuels et al., Science, 2004, 304, 554; Velho et al., Eur. J. Cancer, 2005, 41, 1649-1654), endometrial cancer (Oda et al., Cancer Res., 2005, 65, 10669-10673), gastric carcinomas (Byun et al., M. J. Cancer, 2003, 104, 318-327; Li et al., supra; Velho et al., supra; Lee et al., Oncogene, 2005, 24, 1477-1480), hepatocellular carcinoma (Lee et al., id), small and non-small cell lung cancer (Tang et al., Lung Cancer 2006, J1, 181-191; Massion et al., Am. J. Respir. Crit. Care Med., 2004, 170, 1088-1094), thyroid carcinoma (Wu et al., J. Clin. Endocrinol. Metab., 2005, 90, 4688-4693), acute myelogenous leukemia (AML) (Sujobert et al., Blood, 1997, 106, 1063-1066), chronic myelogenous leukemia (CML) (Hickey et al., J. Biol. Chem., 2006, 281, 2441-2450), and glioblastomas (Hartmann et al., Acta Neuropathol (Berl), 2005, 109, 639-642; Samuels et al., supra).
mTOR is a highly conserved serine-threonine kinase with lipid kinase activity and participates as an effector in the PI3K/AKT pathway. mTOR exists in two distinct complexes, mTORC1 and mTORC2, and plays an important role in cell proliferation by monitoring nutrient availability and cellular energy levels. The downstream targets of mTORC1 are ribosomal protein S6 kinase 1 and eukaryotic translation initiation factor 4E-binding protein 1, both of which are crucial to the regulation of protein synthesis. (“Present and future of PI3K pathway inhibition in cancer: perspectives and limitations” Current Med. Chem., 2011, 18, 2647-2685).
Knowledge about consequences of dysregulated mTOR signaling for tumorigenesis comes mostly from studies of pharmacologically disruption of mTOR by repamycin and its analogues such as temsirolimus (CCI-779) and everolimus (RAD001). Rapamycin was found to inhibit mTOR and thereby induce G1 arrest and apoptosis. The mechanism of rapamycin growth inhibition was found to be related to formation of complexes of rapamycin with FK-binding protein 12 (FKBP-12). These complexes then bound with high affinity to mTOR, preventing activation and resulting in inhibition of protein translation and cell growth. Cellular effects of mTOR inhibition are even more pronounced in cells that have concomitant inactivation of PTEN. Antitumor activity of rapamycin was subsequently identified, and a number of rapamycin analogues such as temsirolimus and everolimus have been approved by the US Food and Drug Administration for the treatment certain types of cancer.
In view of the important role of PI3Ks and mTOR in biological processes and disease states, inhibitors of these kinases are desirable (“Phosphatidylinositol 3-kinase isoforms as a novel drug targets” Current Drug Targets, 2011, 12, 1056-1081; “Progress in the preclinical discovery and clinical development of class I and dual class I/IV phosphoinositide 3-kinase (PI3K) inhibitors” Current Med. Chem., 2011, 18, 2686-2714).